Piezoelectric device comprising ultrahighly-orientated aluminum nitride thin film and its manufacturing method

ABSTRACT

The present invention has an objective to provide a high performance piezoelectric element in which is formed an aluminum nitride thin film free from hillocks, cracks, and peeling which exhibits superhigh c-axis orientation, by forming a bottom electrode from a W layer with no intervening adhesive layer on a glass or other cheap substrate. The piezoelectric element of the present invention is a piezoelectric element using a superhigh-oriented aluminum nitride thin film characterized in that the piezoelectric element is free from hillocks, cracks, and peeling and includes a stack structure in which a bottom electrode, a piezoelectric body thin film, and a top electrode are sequentially formed on a substrate; the bottom electrode is made of an oriented W layer of which a (111) plane of W is parallel to a surface of the substrate; and the piezoelectric body thin film is formed of a c-axis-oriented aluminum nitride thin film having a rocking curve full width half maximum (RCFWHM) not exceeding 2.5°.

TECHNICAL FIELD

The present invention relates to piezoelectric elements using a thinfilm, provided on a glass or other cheap substrate, in which aluminumnitride has a high c-axis orientation and manufacturing methods of suchelements.

BACKGROUND ART

Aluminum nitride is a promising material in efforts to making morecompact, thinner piezoelectric elements for, among other factors, itshigh fabricability into thin film. However, to utilize aluminum nitrideas an piezoelectric element, the aluminum nitride needs to be orientedonly along the c-axis. The more c-axis-oriented, the more piezoelectric.In addition, to use an aluminum nitride thin film as a piezoelectricelement, the film needs be flanked on top and bottom by electrodes.

Thin films of c-axis-oriented aluminum nitride have been reportedlyproduced on glass and other substrates by various methods (T. Shiosaki,T. Yamamoto, T. Oda, A. Kawabata, Appl. Phys. Lett., 36 (1980) 643).There are also reports about the production on electrode film. Thesealuminum nitride films however have great rocking curve full width halfmaximums (RCFWHMs) of about 3.0° or more, a measure of c-axisorientation, and insufficient piezoelectricity.

Aluminum nitride films with superhigh c-axis orientation (2.5° or lessin RCFWHM) are reported by, for example, F. Engelmark, G. F. Iriarte, I.V. Katardjiev, M. Ottosson, P. Muralt, S. Berg, J. Vac. Sci. Technol. A,19 (2001) 2664. A monocrystal substrate is used as the substrate onwhich a monocrystal or polycrystalline thin film of aluminum nitride isdirectly formed. Therefore, no electrode can be provided between thesubstrate and the aluminum nitride thin film, which renders it difficultto use the film as an piezoelectric element.

Aluminum nitride films show very large inherent internal stress. Whenfabricated on an electrode, the film may create cracks in the electrodesor peel off the substrate together with the electrodes, raising seriousproblems in the applying to piezoelectric elements.

The present invention has an objective to provide a high performancepiezoelectric element from an aluminum nitride thin film, with nohillocks or cracks, which does not peel off and exhibits superhighc-axis orientation, by forming a bottom electrode from a W layer on aglass or like cheap substrate with no intervening adhesive layer.

The present invention has another objective to provide a highperformance piezoelectric element from an aluminum nitride thin filmwhich exhibits similar superhigh c-axis orientation, by selecting asuitable material for the surface layer of the bottom electrode in theformation of the bottom electrode which is a stack body containing notonly a W layer, but also an adhesive layer. Specific stack structures ofthe bottom electrode will be also proposed.

The present invention provides an easy and cheap method of manufacturinga piezoelectric element based on the foregoing aluminum nitride thinfilm whereby the aluminum nitride thin film is given superhigh c-axisorientation with the occurrence of hillocks, cracks, and peeling beingprevented through the control of particle shape.

The present invention has a further objective to achieve an equivalentlevel of performance with a cheap glass substrate to that with amonocrystal substrate.

The present invention has still another objective to provide amanufacturing method whereby the bottom electrode is formed by r.f.plasma-assisted sputtering to impart superhigh c-axis orientation to thealuminum nitride thin film.

DISCLOSURE OF INVENTION

A piezoelectric element using a superhigh-oriented aluminum nitride thinfilm in accordance with the present invention is characterized in that:the piezoelectric element is free from hillocks, cracks, and peeling andincludes a stack structure in which a bottom electrode, a piezoelectricbody thin film, and a top electrode are sequentially formed on asubstrate; the bottom electrode is made of an oriented W layer of whicha (111) plane of W is parallel to a surface of the substrate; and thepiezoelectric body thin film is formed of a c-axis-oriented aluminumnitride thin film having a rocking curve full width half maximum(RCFWHM) not exceeding 2.5°.

Another piezoelectric element using a superhigh-oriented aluminumnitride thin film in accordance with the present invention ischaracterized in that: the piezoelectric element is free from hillocks,cracks, and peeling and includes a stack structure in which a bottomelectrode, a piezoelectric body thin film, and a top electrode aresequentially formed on a substrate, the bottom electrode containing as abottom layer an adhesive layer adhering to the substrate; the bottomelectrode is made of a stack body; the stack body has a surface layermade of a metal layer having an electronegativity of around 1.4 and suchan orientation that a crystal plane of a metal having an identicalatomic configuration to an atomic configuration of the (001) plane ofaluminum nitride and an almost equal atomic distance to an atomicdistance on the (001) plane is parallel to a surface of the substrate;and the piezoelectric body thin film is formed of a c-axis-orientedaluminum nitride thin film having a RCFWHM not exceeding 2.5°.

Another piezoelectric element using a superhigh-oriented aluminumnitride thin film in accordance with the present invention ischaracterized in that: the piezoelectric element is free from hillocks,cracks, and peeling and includes a stack structure in which a bottomelectrode, a piezoelectric body thin film, and a top electrode aresequentially formed on a substrate, the bottom electrode containing as abottom layer an adhesive layer adhering to the substrate; the bottomelectrode is made a stack body containing as a surface layer such anoriented W, Pt, Au, or Ag layer that a (111) plane of W, Pt, Au, or Agis parallel to a surface of the substrate; and the piezoelectric bodythin film is formed of a c-axis-oriented aluminum nitride thin filmhaving a RCFWHM not exceeding 2.5°.

A method of manufacturing a piezoelectric element using asuperhigh-oriented aluminum nitride thin film in accordance with thepresent invention is characterized by including the sequential steps of:forming a bottom electrode on a substrate from such an oriented W layerthat a (111) plane of W is parallel to a surface of the substrate bysputtering at a temperature from room temperature to a low temperatureat which no spaces develop between W particles; and forming apiezoelectric body thin film of a c-axis-oriented aluminum nitride thinfilm having a RCFWHM not exceeding 2.5° on the bottom electrode; andforming a top electrode on the piezoelectric body thin film.

Another method of manufacturing a piezoelectric element using asuperhigh-oriented aluminum nitride thin film in accordance with thepresent invention is characterized by including the sequential steps of:in forming, on a substrate, a bottom electrode of a two- or more-layeredstack structure including an adhesive layer adhering to the substrate,firstly depositing the adhesive layer by sputtering at a temperaturefrom room temperature to a low temperature at which no spaces developbetween particles and then depositing as a surface layer of the bottomelectrode a metal layer by sputtering at a temperature from roomtemperature to a low temperature at which no spaces develop betweenparticles so that the metal layer exhibits such orientation that acrystal plane of a metal is parallel to a surface of the substrate, byusing such a metal having an electronegativity of around 1.4 that acrystal plane of the metal has an identical atomic configuration to anatomic configuration on a (001) plane of aluminum nitride and an almostequal atomic distance to an atomic distance on the (001) plane; forminga piezoelectric body thin film of a c-axis-oriented aluminum nitridethin film having a RCFWHM not exceeding 2.5° on the bottom electrode;and forming a top electrode on the piezoelectric body thin film.

Another method of manufacturing a piezoelectric element using asuperhigh-oriented aluminum nitride thin film in accordance with thepresent invention is characterized by including the sequential steps of:in forming, on a substrate, a bottom electrode of a two- or more-layeredstack structure including an adhesive layer adhering to the substrate,firstly depositing the adhesive layer by sputtering at a temperaturefrom room temperature to a low temperature at which no spaces developbetween particles and then depositing as a surface layer an oriented W,Pt, Au, or Ag layer that a (111) plane of W, Pt, Au, or Ag is parallelto a surface of the substrate by sputtering at a temperature from roomtemperature to a low temperature at which no spaces develop betweenparticles; forming a piezoelectric body thin film of a c-axis-orientedaluminum nitride thin film having a RCFWHM not exceeding 2.5° on thebottom electrode; and forming a top electrode on the piezoelectric bodythin film.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical microscopic photograph (image) of the surface of analuminum nitride thin film on a W thin film.

FIG. 2(a) and FIG. 2(b) are optical microscopic photographs (images) ofthe surfaces of aluminum nitride thin films on a Ti/Au thin film (×800)and on an Al—Si thin film (×50) respectively.

FIG. 3(a) and FIG. 3(b) are optical microscopic photographs (images) ofthe surfaces of aluminum nitride thin films on a Cr thin film (×500) andon a Ni thin film ×500) respectively.

FIG. 4 is a drawing showing the relationship between the orientation ofan aluminum nitride thin film and that of a bottom electrode thin film.

FIG. 5 is a drawing showing the relationship between the orientation ofan aluminum nitride thin film and the electronegativity of a bottomelectrode thin film.

FIG. 6(a) and FIG. 6(b) are photographs of atomic force microscope (AFM)images of the surfaces of aluminum nitride thin films on a Ti/Pt thinfilm and on a Cr/Pt thin film respectively.

FIG. 7 is a conceptual drawing illustrating stress acting on a thinfilm.

FIG. 8(a) and FIG. 8(b) are photographs of atomic force microscope (AFM)images of the surfaces of Pt thin films on a Ti thin film and on a Crthin film respectively.

FIG. 9(a) to FIG. 9(c) are photographs of atomic force microscope (AFM)images of the surfaces of aluminum nitride thin films on Ti/Pt thinfilms processed at different temperatures, i.e., room temperature, 300°C., and 400° C. respectively.

FIG. 10(a) to FIG. 10(c) are photographs of atomic force microscope(AFM) images of the surfaces of Pt thin films on Ti/Pt thin filmsprocessed at different temperatures, i.e., room temperature, 300° C.,and 400° C.

FIG. 11 is a load vs. output graph.

FIG. 12 is a microscope photograph (image) of a scratch.

FIG. 13 is graph showing the relationship between pressure on apiezoelectric element and the electric charge it stores at differentRCFWHMs.

BEST MODE FOR CARRYING OUT THE INVENTION

The following will describe the present invention in detail by way ofembodiments examples, which are by no means limiting the presentinvention.

A piezoelectric element in accordance with the present inventionincludes a stack structure in which a bottom electrode, a piezoelectricbody thin film, and a top electrode formed sequentially on a substrate.A premise here is that the piezoelectric element is free from hillocks,cracks, or peeling in the present invention, because hillocks, cracks,and peeling, if having occurred with the bottom electrode, thepiezoelectric body thin film, or the top electrode, seriously damage thereliability of the piezoelectric element.

In the present invention, the piezoelectric element is fabricated on asubstrate. Thus its applications include pressure sensors and surfaceacoustic wave filters. In addition, the piezoelectric element can haveimproved sensitivity.

The substrate used in the present invention may be either a monocrystalsubstrate, such as a sapphire substrate, or a non-monocrystal substrate,such as a glass substrate, a polycrystalline ceramic substrate, a metalsubstrate, and a resin substrate. The present invention is applicable tomonocrystal substrates. The invention is also applicable tonon-monocrystal substrates, forming a film of aluminum nitride (AlN)with superhigh c-axis orientation on such substrates. Notably, theinvention is expected to contribute to the production of piezoelectricelements at low cost.

The bottom electrode may be either a single metal layer or a stack bodycontaining layers including an adhesive layer which adheres to thesubstrate.

The bottom electrode, if it is a single metal layer, is preferablyfabricated from a W layer oriented in such a manner that the (111) planeof the W is parallel to the substrate surface.

If the bottom electrode is a stack body including an adhesive layer, themetal constituting the surface layer of the stack body preferably has anelectronegativity between 1.3 and 1.5 inclusive, more preferably anelectronegativity of around 1.4. On top of these conditions, the metalpreferably has a crystal plane on which the atoms show the sameconfiguration with almost the same atomic distances as those on a (001)plane of aluminum nitride, because that metal well matches the (001)plane of aluminum nitride. Specifically, the crystal plane of such ametal does not differ from the (001) plane of aluminum nitride inlattice constant, allowing the aluminum nitride to grow withoutdeforming. In addition, the surface layer of the bottom electrode ispreferably made of a such a metal layer oriented in such a manner thatthe crystal plane of such a metal is parallel to the substrate surface.

A concrete example of a bottom electrode which is a stack body includingan adhesive layer is a stack body containing a W, Pt, Au, or Ag surfacelayer oriented in such a manner that the (111) plane of the W, Pt, Au,or Ag is parallel to the substrate surface, because the (111) plane ofW. Pt, Au, and Ag well matches the (001) plane of aluminum nitride.Specifically, the (111) plane of W, Pt, Au, and Ag does not differ fromthe (001) plane of aluminum nitride in lattice constant.

Other concrete, preferred examples are Ti/Pt and Cr/Pt double layer. Thenotation “A/B” indicates that the metal A sits on the substrate, thatis, the first layer, and that the metal B sits on the metal A, that is,the second layer. Further examples include Ti/Pt/Au, Ti/Ni/Au, andCr/Ni/Au triple layer. The notation “A/B/C” indicates that the metal Asits on the substrate, that is, the first layer, that the metal B sitson the metal A, that is, the second layer, and that the metal C sits onthe metal B, that is, the third layer.

With no intervening adhesive layer, the Pt, Au, or Ag bottom electrodegrown on the substrate is likely to peel off and develop cracks understress. The provision of an intervening adhesive layer effectivelyprevents peeling and the occurrence of cracks and hillocks. Besides, theadhesive layer enhances orientation on the (111) plane of Pt, Au, andAg, thereby enabling an aluminum nitride thin film to form with superc-axis orientation.

The piezoelectric body thin film of the present invention is made of asuper-c-axis-oriented aluminum nitride thin film with a rocking curvefull width half maximum (RCFWHM) of 2.5° or less. Rocking curvemeasurement gives the deviation and range of the orientation of thecrystal plane. Referring to FIG. 13, there exists a correlation betweenthe RCFWHM and the electric charge stored in a piezoelectric element.The piezoelectric element stores more electric charge and shows betterperformance, with smaller RCFWHMs. In the present invention, thesuper-c-axis-oriented aluminum nitride thin film is defined as thealuminum nitride thin film with a RCFWHM of 2.5° or less.

The top electrode of the present invention can be made up a metal, suchas Al, Pt, Au, and Ag; an alloy primarily of these metals; a conductiveoxide, such as ITO, iridium dioxide, ruthenium dioxide, rheniumtrioxide, and LSCO (La_(0.5)Sr_(0.5)CoO₃); or a conductive nitride, suchas tantalum nitride. These are mere examples, and any conductivesubstance may be used provided that the substance adheres well to thealuminum nitride thin film and causes no substantial stress.

Now, a manufacturing method for the piezoelectric element of the presentinvention will be described. First, the substrate may be selected frommonocrystal substrates, polycrystalline substrates, and amorphoussubstrates. In the present invention whereby a superhigh-c-axis-orientedaluminum nitride thin film can be grown regardless of the kind ofsubstrate, however, it is preferable to select a polycrystallinesubstrate or an amorphous substrate. Glass substrates, especially,quartz glass substrates, are preferred.

In the present invention, physical vapor deposition (PVD) is used,because a metal is used to form the bottom electrode regardless ofwhether the electrode contains or does not contain an adhesive layer.Example of PVD include: vacuum vapor deposition, such as resistancethermal vapor deposition and electron beam thermal vapor deposition;various sputtering, such as DC sputtering, high frequency sputtering,r.f. plasma-assisted sputtering, magnetron sputtering, ECR sputteringand ion beam sputtering; various ion plating, such as high frequency ionplating, activated vapor deposition, and arc ion plating; molecular beamepitaxy; laser abrasion; ionized cluster beam vapor deposition; and ionbeam vapor deposition. The bottom electrode is grown of a predeterminedmetal or alloy by these processes, preferably, by sputtering, andespecially preferably, by r.f. plasma-assisted sputtering. A suitablevapor deposition process is selected from these methods, depending on avapor-deposited substance.

The bottom electrode is deposited at low temperatures, preferably atroom temperature. The temperature should however not be so low thatspaces develop between metal particles constituting the bottomelectrode. Spaces between particles would render the electrode moresusceptible to cracks and peeling and a short circuit more likelybetween the top and bottom electrodes.

Above those temperatures at which spaces develop between particles, theparticles may grow during film deposition, which would smooth out thefine structures of the bottom electrode. When this is the case, a shortcircuit is hardly likely. The electrode may be deposited at suchtemperatures that the particle growth eliminates spaces betweenparticles and produces flat fine structures.

Deposition conditions are, for example, a pressure of 1.0×10⁻¹ Pa, anitrogen gas partial pressure ratio of 0%, no substrate heating, and atarget introduction electric power of 200 W. The film thickness isvaried depending on the material. All these conditions may be suitablyaltered.

To form a bottom electrode of a two- or more-layered stack structureincluding an adhesive layer adhering to the substrate on the substrate,first, the adhesive layer is deposited by sputtering at a lowtemperature at which no spaces develop between particles, preferably atroom temperature. Next, on the adhesive layer is formed an electrodesurface which well matches the (001) plane of aluminum nitride. Theelectrode surface is, for example, a metal with an electronegativity ofaround 1.4 having a crystal plane on which the atoms show the sameconfiguration with almost the same atomic distances as those on a (001)plane of aluminum nitride. Using this metal, a metal layer where thecrystal plane of the metal is parallel to the substrate surface isdeposited on the surface layer of the bottom electrode by sputtering sothat the layer is oriented, to complete the bottom electrode. In thiscase, the deposition temperature is, as mentioned earlier, low, and atthat temperature, no spaces develop between metal particles constitutingthe bottom electrode, preferably room temperature.

Under these circumstances, the surface layer may be formed by sputteringa W, Pt, Au, or Ag layer oriented in such a manner that the (111) planeof the W, Pt, Au or Ag is parallel to the substrate surface. In thiscase, the deposition temperature is low, and at that temperature nospaces develop between the metal particles, preferably, roomtemperature.

When forming a bottom electrode containing an adhesive layer, the bottomelectrode is again preferably deposited by r.f. plasma-assistedsputtering.

Depositing the bottom electrode at a low temperature provides a suitablesurface layer to orient aluminum nitride and eliminates differences inthermal expansion for lower stress, allowing for no cracks, hillocks, orpeeling.

Next, a piezoelectric body thin film is deposited of a c-axis-orientedaluminum nitride thin film with a 2.5° or less RCFWHM on the bottomelectrode. The step is performed by PVD, preferably by sputtering. Uponcompletion of the fabrication of the bottom electrode, the metal whichis the electrode material constituting the surface layer of the bottomelectrode is oriented in such a manner that the crystal plane on whichthe atoms show the same configuration with almost the same atomicdistances as those on the (001) plane of aluminum nitride is parallel tothe substrate. Therefore, a bed surface forms which is equivalent tosapphire and other monocrystals. A super-c-axis-orientated aluminumnitride thin film is obtained by forming a thin film on the surfacelayer of the bottom electrode by PVD whereby the aluminum nitride is thetarget. Deposition conditions here are, for example, a pressure of1.3×10⁻¹ Pa, a nitrogen gas partial pressure ratio of 60%, a substratetemperature of 300° C., and a target introduction electric power of 200W. The film thickness is 2000 nm. These conditions may be suitablyaltered.

Then, a top electrode is formed on the piezoelectric body thin film. Thetop electrode material is formed by either PVD or CVD. A suitably vapordeposition process is selected depending on the vapor-depositedsubstance.

EXAMPLES

The following will describe the present invention in more detail by wayof examples. Chemical elements are denoted by symbols in the presentinvention.

Fabrication of High Orientation Thin Film

The electrical properties of an aluminum nitride thin film, includingelectromechanical coupling factor, is known to vary greatly depending oncrystal orientation. Accordingly, to obtain a high orientation AlN thinfilm, effects of a bottom electrode and stack effects of a bottomelectrode were examined.

Example 1 Effects of Bottom Electrode

Most researches into AlN thin film growth on a conductor have beenconducted around the improvement of iron's corrosion resistance and thegrowth on Al electrodes for use in a surface acoustic wave (SAW) filter.Only a small number of researches have been conducted into the growth onother conductors. According to reports, the highest orientation AlN thinfilm so far is fabricated on an Au thin film on a glass substrate andexhibits a rocking curve full width half maximum of 3°. Accordingly, tofind the high orientation AlN thin film on a bottom electrode, an AlNthin film was fabricated on 20 types of conductor thin films, to observethe effects of a crystal structure to the AlN thin film. Most of the 20types of conductor thin films were processed by sputtering at roomtemperature. Table 1 shows XRD measurements on the obtained AlN thinfilms. The substrates were all made of glass.

Specifically, each bottom electrode specimen was deposited on a quartzglass substrate (20 mm×20 mm×1.1 mm). Deposition conditions were apressure of 1.0×10⁻¹ Pa, a nitrogen gas partial pressure ratio of 0%, nosubstrate heating, and a target introduction electric power of 200 W.The film thickness was varied depending on the material. The depositionconditions for aluminum nitride were a pressure of 1.3×10⁻¹ Pa, anitrogen gas partial pressure ratio of 60%, a substrate temperature of300° C., a target introduction electric power of 200 W, and a filmthickness of 2000 nm. TABLE 1 XRD measurements of AlN thin films onconductive body thin films (002) peak full width Conductive RCFWHMIntegrated (002) c-axis half body (°) peak intensity length (A) maximum(°) Au—Cr 5.75  530814 4.980 0.25 Al—Cu 7.24  263574 4.986 0.28 Al—Si5.67  401799 4.986 0.26 Al—Cu—Si 6.76  276111 4.986 0.27 Al## 5.67 401799 4.986 0.26 Ni 9.43  148275 4.982 0.28 Cr 10.34  51359 4.988 0.25Ta 4.49  379861 4.988 0.28 Nb 4.17  280364 4.990 0.28 Fe 4.68  2270474.990 0.29 W 2.14 5434083 4.982 0.19 Ti/Pt 2.06 6554966 4.982 0.18 Ti/Ir6.53  225899 4.986 0.27  10381 (103) Ti/Au 1.57 ** 4.982 0.16 Ti/Pd 4.01 728326 4.982 0.23 Ti/Rh 4.60 1102793 4.986 0.18 Ti/Ag 2.44 21369654.982 0.19 Ti/Mo 7.33  246529 4.982 0.27   3858 (103) Ti/Ru** — — — —** Outside instrument's scale**Peeled off##Deposited in vacuum

Al—Si, Ni, Cr, and like materials are often used with semiconductors; ifthey can be used to form an electrode, they come in handy when thealuminum nitride piezoelectric element is integrated into asemiconductor device. Disappointedly, however, these materials had ahigh RCFWHM and developed many cracks. Pt and Au, if formed directly ona quartz substrate, did not adhere to the substrate well, resulting infrequent peeling. To address the problem, an adhesive layer of Ti, Cr,etc. was inserted. The measurements in Table 1 demonstrate that AlN thinfilms with superhigh orientation giving a rocking curve full width halfmaximum of about 2° were obtained on W, Ti/Pt, Ti/Au, and Ti/Ag.

For double-layer bottom electrodes, the left side element sits on thesubstrate, i.e. the first layer. For example, the notation, “Ti/Pt,”indicates that Ti is the first layer and Pt is the second layer. Fortriple-layer bottom electrode, the notation is in the form “firstlayer/second layer/third layer.”

Mass production with excellent microscopic crystal structure becomesdifficult if macroscopic problems, such as cracks and peeling, occur.Accordingly, the AlN thin film surfaces were observed under an opticalmicroscope. Results are shown in FIGS. 1, 2(a), 2(b), 3(a), and 3(b).FIG. 1 is an optical microscopic photograph (×800) of the surface of anAlN thin film with superhigh orientation and high crystallinity whichwas formed on a W thin film. The surfaces of the AlN thin films on W andTi/Pt were smooth and uniform with no cracks or peeling observed at allas shown in FIG. 1.

In contrast, hillocks and large cracks were observed on the surface ofthe AlN thin films on Ti/Au and Ti/Ag as shown in FIG. 2(a). To exploitthe existing semiconductor technology, it is better to use Al—Si.However, high orientation could not given to the AlN thin film on aAl—Si thin film, and intense cracking was observed as shown in FIG.2(b). FIG. 3(a) and FIG. 3(b) are optical microscopic photographs of thesurfaces of AlN thin films on a Cr thin film and a Ni thin film forcomparison. Numerous cracks and some pinholes were observed in the AlNthin films on a Cr thin film, a Ni thin film, etc. developed as shown inFIG. 3(a) and FIG. 3(b). These results demonstrate that the AlN thinfilms on W and Ti/Pt exhibited high orientation and high crystallinityand developed no cracks, etc. Therefore, the W or Ti/Pt thin film formexcellent bottom electrode thin films.

To find out why the AlN thin film has an excellent crystal structure onW, Ti/Pt, Ti/Au, and Ti/Ag, we examined the relationship between theorientation of bottom electrode thin films and the orientation of theAlN thin films on those bottom electrode thin films and the relationshipbetween the electronegativity of bottom electrode materials and theorientation of the AlN thin films on that bottom electrode thin films.Results are shown in FIG. 4 and FIG. 5. The orientation of the AlN thinfilm tended to increase with increasing orientation of the bottomelectrode thin film. Also, the orientation of the AlN thin film was highwhen the electronegativity of the bottom electrode material is around1.4. These results demonstrate that the AlN thin film on a conductorexhibited an excellent crystal structure when fabricated on a conductorwith an electronegativity of around 1.4.

Example 2 Effects of Stacked Bottom Electrode

There has been no single report on stack effects of a bottom electrodefor the AlN thin film. Accordingly, we examined effects on the AlN thinfilm crystal structure, such as orientation and crystallinity, of twoand three metal thin films being stacked together.

We examined bottom electrodes containing Pt, i.e. Ti/Pt and Cr/Pt, whichhad high orientation and high crystallinity. The substrate was a quartzglass (20 mm×20 mm×1.1 mm). The deposition conditions for the bottomelectrode were a pressure of 1.0×10⁻¹ Pa, a nitrogen gas partialpressure ratio of 0%, no substrate heating, and a target introductionelectric power of 200 W. The film thickness was varied depending on thematerial. In addition, the deposition conditions for the aluminumnitride were a pressure of 1.3×10⁻¹ Pa, a nitrogen gas partial pressureratio of 60%, a substrate temperature of 300° C., and a targetintroduction electric power of 200 W. The film thickness was 2000 nm.

Table 2 shows XRD results for the AlN thin films fabricated on thesethin films. The change of the bottom segment from Ti to Cr greatlyreduced the rocking curve full width half maximum of the AlN thin filmfrom 2.06° to 0.40° and almost doubled the peak integrated intensity ofthe AlN (002) plane. The AlN (001) plane-to-plane distance (c-axislength) was 4.980×10⁻⁸ m (4.980 angstroms). The AlN (001) plane-to-planedistances on the thin films were 4.982×10⁻⁸ m (4.982 angstroms) and4.984×10⁻⁸ m (4.984 angstroms). It was inferred that the c-axis became alittle longer and a compressive stress occurred perpendicular to thec-axis. These results demonstrate that the Cr thin film, although only afew tens of nanometers thick, greatly affected the crystal structure ofthe AlN thin film formed thereon. To the best of knowledge of theinventors, the only reported superhigh orientation AlN thin film with arocking curve full width half maximum of 0.40° is made on a substrate ofα-Al₂O₃ monocrystal (sapphire) at substrate temperatures of 500° C. orhigher. This is the first ever superhigh orientation AlN thin film madeon a glass substrate at such a low temperature as 300° C. TABLE 2 XRDmeasurements of AlN thin films on stacked thin films containing PtIntegrated c-axis (111) peak full Stack RCFWHM (002) length width halfstructure (nm) (°) peak intensity (A) maximum (°) Ti(20)/Pt(200) 2.06 6554966 4.982 0.18 Cr(40)/Pt(200) 0.40 10931000 4.984 0.12

To find out effects of the bottom segment layers on the orientation andcrystallinity of AlN thin films, the surfaces of the AlN thin films wereobserved under an atomic force microscope (AFM) which is a suitable toolto observe the surface of a very small area. Results are shown in FIG.6(a) and FIG. 6(b). On a Ti/Pt thin film, the surface roughness (Ra) ofthe AlN thin film was 2.95 nm, with the surface formed by particles witha diameter of about 30 to 50 nm. See FIG. 6(a). In contrast, the surfaceroughness (Ra) of the AlN thin film on a Cr/Pt thin film which had highorientation was relatively low at 1.69 nm, with the surface formed bysecondary particles (combined particles). See FIG. 6(b). Despite thatthe Cr/Pt sample was made of relatively large particles, the surfaceroughness was low, and the surface was flat. These results demonstratethat the bottom segment metal thin film had large effects on theorientation and crystallinity of the AlN thin film, that is, on thegrowth of the AlN thin film and that the selection of a bottom segmentmetal thin film material was very important to obtain an AlN thin filmwith excellent crystallinity.

To further examine the effects of the bottom segment Cr thin film, thecrystal structure of the intervening Pt thin film was examined by XRD.Results are shown in Table 3. When the Ti bottom segment was replaced bya Cr bottom segment, the Pt thin film exhibited an increased rockingcurve full width half maximum and a greatly decreased integratedintensity. In addition, since the Pt (111) plane-to-plane distance(lattice constant) was 2.2650×10⁻⁸ m (2.2650 angstroms), the differencefrom the lattice constant of the Pt thin film became smaller. Theseresults demonstrate that the use, of a Cr bottom segment reduces theorientation and crystallinity of the Pt thin film. Initially, we hadpresumed that the AlN thin film on the Pt thin film would exhibit highorientation and high crystallinity because of the high orientation andhigh crystallinity of the Pt thin film, however, it was understood thatthe orientation and crystallinity of the Pt thin film were not majorfactors dictating the crystal structure of the AlN thin film. There hadto be different major factors dictating the crystal structure of AlNthin film. In addition, as shown in FIG. 7, from the changes of latticeconstant, it is understood that the AlN thin film was under compressivestress and the Pt was under tensile stress. TABLE 3 XRD measurements ofPt thin films after fabrication of AlN thin films Integrated Lattice(002) peak full Stack RCFWHM (111) constant width half structure (nm)(°) peak intensity (A) maximum (°) Ti/Pt 2.49 2380732 2.245 0.28 Cr/Pt8.52  174918 2.255 0.36

To find out major factors dictating the crystal structure of the AlNthin film, the both Pt thin films were observed under a atomic forcemicroscope (AFM). Results are shown in FIG. 8(a) and FIG. 8(b). On theTi thin film, the surface roughness (Ra) of the Pt thin film was 1.99nm, with the surface formed by uniform particles with a diameter ofabout 50 nm (FIG. 8(a)). In contrast, on the Cr thin film, the surfaceroughness (Ra) of the Pt thin film was 4.09 nm. Some particles with adiameter of about 200 nm were observed, and the surface was not uniform.Where there were no particles, the surface was very flat. See FIG. 8(b).From these results, we inferred that the improved orientation andcrystallinity of the AlN thin film with a Cr bottom segment was causedby the drop in the surface roughness, that is, the improved flatness, ofthe Pt thin film.

Next, we examined stack effects of bottom electrodes containing Au, i.e.Ti/Au, Ti/Pt/Au, Ti/Ni/Au, and Cr/Ni/Au. Results are shown in Table 4.The rocking curve full width half maximum was substantially around 1.6°for all the samples. There were no significant differences. Nosignificant differences were found either for the (002) peak integratedintensity. Therefore, no stack effects were observed with theAu-containing thin film in contract to the Pt-containing thin film.TABLE 4 Effects of thin film stack structure containing Au layer on AlNthin films Integrated c-axis (002) peak full Stack RCFWHM (002) lengthwidth half structure (nm) (°) peak intensity (A) maximum (°) Ti/Au1.57 * 4.982 0.16 Ti/Pt/Au 1.57 7357359 4.986 0.15 Ti/Ni/Au 1.56 66660204.986 0.17 Cr/Ni/Au 1.77 7352273 4.982 0.16* Immeasurable

To find out why no stack effects were observed with Au-containingelectrodes, the crystal structure of Au thin film samples were examinedby XRD. Results are shown in Table 5. TABLE 5 XRD measurements of Authin film stack structure Integrated Lattice (111) peak full StackRCFWHM (111) constant width half structure (nm) (°) peak intensity (A)maximum (°) Ti/Au * * * * Ti/Pt/Au 1.62 7158859 2.334 0.27 Ti/Ni/Au 4.361262213 2.325 0.31 Cr/Ni/Au 6.60  651529 2.317 0.26* Outside instrument's scale

The peak rocking curve full width half maximum of the Au (111) planevaried greatly from 1.62 to 6.60°. The integrated intensity also variedgreatly, by a whole order of magnitude. In addition, the Au (111)plane-to-plane distance was 2.3550×10⁻⁸ m (2.3550 angstroms). The Aulattice constant was, however, short for all the electrodes. The latticeconstant for Cr/Ni/Au was the shortest at 2.317×10⁻⁸ m (2.317angstroms). This was presumably due to large tensile stress acting onthe Au thin film. These results demonstrate that AlN thin films onAu-containing electrodes exhibited high orientation and highcrystallinity regardless of the orientation and crystallinity of the Au.In this case, the flatness of the thin film surface was again inferredto be a major factor. In addition, the AlN thin film is undercompressive stress, and the Au thin film is under tensile stress, asshown in FIG. 7.

Similar comparison was made for Al and Al-containing (Cr/Al) electrodes.Results are shown in Table 6. The rocking curve full width half maximumdropped from 5.88° to 2.57°. The integrated intensity increased from600,000 to 2,090,000 (cps). These results demonstrate that Al-containingbottom segment thin films greatly affect the orientation andcrystallinity of the AlN thin film, that is, the growth, of the AlN thinfilm. Therefore, to obtain an AlN thin film with excellentcrystallinity, the selection of material for the bottom segment metalthin film should be given due consideration. TABLE 6 Effects of thinfilm stack structure containing Al layer on AlN thin films Integratedc-axis (002) peak full Stack RCFWHM (002) length width half structure(nm) (°) peak intensity (A) maximum (°) Cr/Al EB 2.57 2091808 4.986 020vapor deposition Al (AIST) 5.88  605246 4.982 0.26

In examples 1, 2, the effects of the bottom electrode and the stackingthereof were examined for the purpose of fabricating a superhighorientation AlN thin film. The examination revealed that theorientation, crystallinity, etc. of the AlN thin film undergosubstantial changes depending on the type of bottom electrode and thestack structure of the bottom electrode. In other words, the examinationof the effects of the bottom electrode led to successful fabrication ofhigh orientation AlN thin films on W, Ti/Au, Ti/Ag and Ti/Pt thin films.However, hillocks and large cracks were observed on the AlN thin filmsformed on Ti/Au and Ti/Ag thin films. Ti/Au and Ti/Ag turned out unfitfor bottom electrode material. In contrast, the surface of the AlN thinfilms on W and Ti/Pt were uniform and almost free from cracks andpeeling, which demonstrated that W or Ti/Pt were suitable bottomelectrode materials. The bottom electrode materials commonly tend toexhibit improved orientation and crystallinity of the AlN thin film withincreased orientation and crystallinity of the bottom electrode. Inaddition, materials having an electronegativity of around 1.4 aresuitable electrode materials.

Examining the stack effects of the bottom electrode revealed that whenthe thin film contains Pt, the bottom segment thin film material greatlyaffect the orientation and crystallinity of the AlN thin film, that is,the growth of the AlN thin film. To obtain an AlN thin film withexcellent crystallinity, the bottom segment substance should beoptimized. It is inferred that Cr bottom segments improve theorientation and crystallinity of the AlN thin film because of thereduced surface roughness (increased flatness) of the Pt thin film. ForAl-containing thin films, large effects were again observed due to Cr.For Au-containing thin films, however, little change was observed.

These results show that AlN thin films with superhigh orientation(rocking curve full width half maximum=0.4°) are obtainable on a glasssubstrate at low temperatures, if the W thin film, Ti/Pt stack thinfilm, or Cr/Pt stack thin film is used for the bottom electrode.

Prevention of Cracks and Short-Circuits

Cracks and pinholes are possible causes of the short-circuiting of thetop and bottom electrodes. To prevent the occurring of these cracks andpinholes, the effects of the fabrication temperature of the bottomelectrode and those of the fabrication method of the bottom electrodewere examined. In addition, to evaluate the reliability of the AlN thinfilm as sensors and other piezoelectric elements, the adhesion strengthof the AlN thin film was evaluated.

Example 3 Effects of Fabrication Temperature of Bottom Electrode

Possible causes of cracks and peeling are differing thermal expansioncoefficients of the substrate and bottom electrode and the AlN.Accordingly, to reduce the effects of the differing thermal expansioncoefficients for prevention of cracks and peeling, the effects of thefabrication temperature of the bottom electrode was examined. Ti/Pt thinfilms with high orientation were used as bottom electrodes. Ti/Pt bottomelectrodes were fabricated at room temperature, 300° C., and about 400°C. The top electrode was an Al thin film fabricated by vacuum vapordeposition. Three samples were simultaneously fabricated at differenttemperatures, to examine short-circuiting of the samples.

When the bottom electrode was fabricated at room temperature, all thesamples experienced no short circuiting. In contrast, when the bottomelectrode was fabricated at 300° C., one third of the samplesexperienced no short circuiting. When the bottom electrode wasfabricated at 400° C., none of the samples experienced short circuiting.The results demonstrate that the Ti/Pt bottom electrode should befabricated at either room temperature or 400° C. to fabricate sampleswith no short circuiting.

To examine the effects of the fabrication temperature of the bottomelectrode on the crystal structure of the AlN thin film, the crystalstructure of the samples were measured by XRD. Results are shown inTable 7. The higher the fabrication temperature of the Ti/Pt bottomelectrode thin film, the greater the rocking curve full width halfmaximum of the (002) plane of the AlN thin film, and the lower the peakintegrated strength of the (002) plane. No effects of the fabricationtemperature on the c-axis length were observed. C-axis lengths were allabout 4.980×10⁻⁸ m (4.980 angstroms). No internal stress occurred. Theresults demonstrate that the higher the fabrication temperature of theTi/Pt bottom electrode thin film, the lower the orientation andcrystallinity of the AlN thin film. Therefore, the bottom electrode thinfilm should be fabricated at room temperature to obtain an AlN thin filmwith no short circuits with high orientation and high crystallinity.TABLE 7 XRD measurements of AlN thin films on bottom electrode thinfilms fabricated at different temperatures Integrated c-axis (002) peakfull (002) length width half Temperature RCFWHM (°) peak intensity (A)maximum (°) Room 2.18 6554966 4.982 0.18 temperature 300° C. 2.196140362 4.982 0.18 400° C.* 2.68 3379954 4.982 0.21*AlN thin films also fabricated at 400° C.

To further examine the effects of fabricate conditions for the bottomelectrode, the surface shape of the samples were examined under anatomic force microscope. Results are shown in FIG. 9(a) to FIG. 9(c).Measurements of the surface roughness and mean particle diameters areshown in Table 8. Referring to FIG. 9(a) to FIG. 9(c), the higher thefabrication temperature of the Ti/Pt thin film, the greater the size ofthe particles forming the AlN thin film. Many spaces were observedbetween particles for 300° C. However, for 400° C., the particlediameter grew, but no spaces between particles were observed. Inaddition, the surface roughness also tended to increase with increasingfabrication temperature and reached 17.9 nm at 400° C. These resultsdemonstrate that the fabricated samples do not experience shortcircuiting at room temperature and 400° C., because the formed filmincluded no spaces and is of very fine structure. TABLE 8 Surfaceroughness and mean particle diameter of AlN thin films on electrodesfabricated at different temperatures Surface roughness Mean particleTemperature (Ra) (nm) diameter (nm) Room temperature 1.6 28.5 300° C.3.1 52.1 400° C. 17.9 —

To examine direct effects of the fabrication temperature of the bottomelectrode on the Pt thin film, the crystal structure of the Pt thin filmwere measured by XRD. Results are shown in Table 9. The higher thefabrication temperature, the lower the rocking curve full width halfmaximum of the (111) plane of the Pt thin film, and the lower theintegrated intensity. No peak was observed with the Pt thin film for400° C. These results demonstrate that the higher the fabricationtemperature, the lower the orientation and crystallinity of the Pt thinfilm. Therefore, it is inferred that the higher the fabricationtemperature of the bottom electrode, the lower the orientation andcrystallinity of the AlN thin film, because the orientation andcrystallinity of the Pt thin film which is the bottom electrode fallswith fabrication temperature. TABLE 9 XRD measurements of Pt thin filmsafter vapor deposition of AlN thin films Integrated Lattice (111) peakfull (111) constant width half Temperature RCFWHM (° ) peak intensity(A) maximum (° ) Room 2.49 2380732 2.245 0.28 temperature 300° C. 7.15 316850 2.254 0.26 400° C. * * * ** No peak observed for Pt

To understand the surface shape of the Pt thin film samples in detail,an atomic force microscope was used in observation. Results are shown inFIG. 10(a) to FIG. 10(c). In addition, measurements of the surfaceroughness and mean particle diameters of Pt thin films fabricated atdifferent temperatures are shown in Table 10. The higher the fabricationtemperature of the Ti/Pt thin film, the greater the size of theparticles forming the Pt thin film, and the greater crystal theparticles formed. The surface roughness also tended to increase andshowed remarkable increases for 400° C. These results demonstrate thatthe changes in the surface roughness and mean particle diameter of theAlN thin film with the fabrication temperature of the bottom electrodethin film were due to large changes in the surface roughness and meanparticle diameter of the Pt thin film which is the bottom electrode withthe fabrication temperature. Therefore, fabricating the bottom electrodeat room temperature produces smooth-surfaced AlN thin films with noshort circuits which exhibit high orientation and high crystallinity.TABLE 10 Effects of fabrication temperature on surface roughness andmean particle diameter Surface roughness Mean particle Temperature (Ra)(nm) diameter (nm) Room temperature 1.9 65.9 300° C. 2.8 — 400° C. 15.9—

The effects of fabrication temperature on short circuiting were examinedfurther on four other bottom electrode materials (Cr/Pt, Ti/Pt/Au,Ti/Ni/Au, Cr/Ni/Au). Results are shown in Table 11. TABLE 11 Sampleshort tests Bottom electrode Fabrication Sample No. structuretemperature {circle over (1)} {circle over (2)} {circle over (3)} Cr/PtRoom temperature X ◯ ◯ 300 ◯ ◯ ◯ Ti/Pt/Au Room temperature ◯ ◯ ◯ 300 X ◯X Ti/Ni/Au Room temperature X X ◯ 300 X ◯ X Cr/Ni/Au Room temperature ◯◯ ◯ 300 ◯ ◯ ◯◯: No ShortX: Short

The effects of fabrication temperature on short circuiting were examinedalso on a Cr/Pt bottom electrode. Two thirds of the Cr/Pt thin filmsamples fabricated at room temperature experienced no short circuiting.None of those fabricated at 300° C. experienced short circuiting at all.So, in the case of Cr/Pt, fabricating at 300° C. produced samples whichwere unlikely to see short circuiting. This was a different result fromthe case of Ti/Pt. Table 12 shows XRD measurements on AlN thin filmsdiffering in the fabrication temperature of the bottom electrode. In thecase of Cr/Pt, the orientation and crystallinity again dropped when thefabrication temperature reached 300° C. These results were similar tothose of the Ti/Pt case. In addition, no Pt peak was observed for 300°C. From these facts, Cr/Pt particles presumably undergo a differentgrowth from Ti/Pt at 300° C., attaining different results. TABLE 12 XRDmeasurements of AlN thin films on Cr/Pt bottom electrode fabricated atdifferent temperatures Integrated (002) peak full (002) peak c-axiswidth half Temperature RCFWHM (°) intensity length (A) maximum (°) Room0.40 10931000 4.984 0.12 temperature 300° C. 2.11 5789083 4.988 0.17

The effects of the fabrication temperature were also examined onTi/Pt/Au bottom electrodes. All Ti/Pt/Au thin film samples fabricated atroom temperature experienced no short circuiting. One third of thosefabricated at 300° C. saw short circuiting. These results were similarto those of the Ti/Pt case. Table 13 shows XRD measurements on AlN thinfilms differing in the fabrication temperature of the bottom electrode.In the case of Ti/Pt/Au, the orientation and crystallinity again droppedwhen the fabrication temperature reached 300° C. These results weresimilar to those of the Ti/Pt case. In addition, no Au peak was observedfor 300° C. From these facts, it is inferred that short circuitingoccurs at 300° C., presumably because Ti/Pt/Au particles undergo asimilar growth to Ti/Pt, creating spaces between particles.

Table 13 XRD measurements of AlN thin films on Ti/Pt/Au bottom electrodefabricated at different temperatures Integrated c-axis (002) peak fullRCFWHM (002) peak length width half Temperature (°) intensity (A)maximum (°) Room temperature 1.57 7357359 4.986 0.15 300° C. 1.956815766 4.988 0.16

The effects of the fabrication temperature were also examined onTi/Ni/Au bottom electrodes. Two thirds of Ti/Ni/Au thin film samplesfabricated at room temperature experienced no short circuiting. Onethird of samples fabricated 300° C. experienced no short circuiting.Table 14 shows XRD measurements on AlN thin films differing in thefabrication temperature of the bottom electrode. In the case ofTi/Ni/Au, when the fabrication temperature reached 300° C., theorientation dropped, whereas the crystallinity improved. These resultswere different from, for example, Ti/Pt. In addition, no Ni and Au peakswere observed for 300° C. From these facts, it is inferred that shortcircuiting occurs at room temperature and at 300° C., presumably becauseTi/Ni/Au particles undergo a different growth from Ti/Pt, etc., creatingspaces between particles.

Table 14 XRD measurements of AlN thin films on Ti/Ni/Au bottom electrodefabricated at different temperatures Integrated c-axis (002) peak fullRCFWHM (002) peak length width half Temperature (°) intensity (A)maximum (°) Room temperature 1.56 6666020 4.986 0.17 300° C. 1.787598293 4.988 0.16

The effects of the fabrication temperature were also examined onCr/Ni/Au bottom electrodes. No Cr/Ni/Au thin film samples experiencedshort circuiting regardless of the fabrication temperature. Table 15shows XRD measurements on AlN thin films differing in the fabricationtemperature of the bottom electrode. In the case of Cr/Ni/Au, theorientation and crystallinity improved when the fabrication temperaturereached 300° C. These results were different from, for example, Ti/Pt.In addition, no Ni and Au peaks were observed for 300° C. From thesefacts, it is inferred that no short circuiting occurs at roomtemperature and 300° C., presumably because Cr/Ni/Au particles undergo adifferent growth from Ti/Pt, etc., creating a fine structure film.

Table 15 XRD measurements of AlN thin films on Cr/Ni/Au bottom electrodefabricated at different temperatures Integrated c-axis (002) peak fullRCFWHM (002) peak length width half Temperature (°) intensity (A)maximum (°) Room temperature 1.77 7352273 4.982 0.16 300° C. 1.669033786 4.988 0.15

Example 4 Effects of Fabrication Method of Bottom Electrode

Ti/Pt bottom electrode thin films were fabricated using a DC (DC)sputtering device to examine effects of different fabrication methods.Table 16 shows XRD measurements on AlN thin films fabricated ondifferent thin films. The rocking curve full width half maximum wasnarrow, and the integrated intensity was high, with samples fabricatedby r.f. plasma-assisted sputtering (RF). Insulation was also examined.None of the samples fabricated by r.f. plasma-assisted sputteringexperienced short circuiting. Two thirds of those fabricated by DCsputtering experienced no short circuiting. These results show that r.f.plasma-assisted sputtering is better in fabricating Ti/Pt thin films.TABLE 16 XRD measurements of AlN thin films on Ti/Pt thin filmsIntegrated (002) peak full Fabrication (002) peak c-axis width halfprocess RCFWHM (°) intensity length (A) maximum (°) RF 2.06 65549664.982 0.18 DC 2.61 2429798 4.982 0.21DC: Direct current sputteringRF: R.f. plasma-assisted sputtering

To examine causes of the difference, the crystal structure of Pt thinfilms was examined by XRD. Results are shown in Table 17. No significantdifferences were found in the full width half maximum and integratedstrength of the (111) peak. These facts confirmed no effects of thedifferent fabrication methods on the crystal structure of the Pt thinfilm. We assume flatness as a likely cause of effects. TABLE 17 XRDmeasurements of Pt thin films Integrated (111) peak full FabricationRCFWHM (111) peak Lattice width half process (°) intensity constant (A)maximum (°) RF 2.49 2380732 2.245 0.28 DC 2.62 1955266 2.251 0.29DC: Direct current sputteringRF: R.f. plasma-assisted sputtering

Example 5 Evaluation of Thin Film Adhesion Strength

Thin films, if easy to peel off, are very difficult to use as sensorelements. Accordingly, the adhesion strength of AlN thin films needs bemeasured. The adhesion strength of AlN thin films were evaluated byscratch tests with a scan scratch tester (Shimazu SST-101). Aload-cartridge output graph is shown in FIG. 11. A microscopicphotograph (×150) of a scratch is shown in FIG. 12.

Details of scratch test conditions are shown in Table 18. Three scratchtests were conducted. The mean peel-off load was 169.2 mN, the standarddeviation 1.0, and coefficient of variation 0.6%. Measurement wasperformed with good reproducibility. Peel-off loads were readilyobtained from scratch images in the microscope vision (see FIG. 12). Thepeel-off load of 169.2 mN under these measurement conditions is higherthan typical thin film adhesion strength, indicating that the adhesionstrength of the AlN thin film is sufficient for sensor element use.TABLE 18 Measurement conditions for scratch test equipment Cartridge tipdiameter (mm) 15 Full-scale load (mN) 200 Load rate (μm/s) 2 Width (μm)100 Feed rate (μm/s) 10

In examples 3, 4, and 5, cracks and pinholes were possible causes of theshort circuiting of the top and bottom electrodes. To prevent theoccurring of these cracks and pinholes, the effects of the fabricationtemperature of the bottom electrode and those of the fabrication methodof the bottom electrode were examined. If a Ti/Pt thin film was used asthe bottom electrode, fabrication at room temperature or 400° C.produced samples with no short circuits. The samples fabricated at roomtemperature or 400° C. experienced no short circuiting, because a finestructure film was formed without there being any spaces betweenparticles. In addition, the orientation and crystallinity of the AlNthin film decreased with an increase in the fabrication temperature ofthe bottom electrode, presumably because the orientation andcrystallinity of the Pt thin film which was the bottom electrodedecreased with fabrication temperature. These results indicate thatfabricating the bottom electrode at room temperature producessmooth-surfaced AlN thin films with no short circuits which exhibit highorientation and high crystallinity.

The effects of fabrication temperature on short circuiting were examinedfurther on four other bottom electrode materials (Cr/Pt, Ti/Pt/Au,Ti/Ni/Au, Cr/Ni/Au). The effects of the fabrication temperature turnedout differing depending on the stack structure of the electrode thinfilm. None of Cr/Pt samples fabricated at 300° C., none of Ti/Pt/Ausamples at room temperature, and none of Cr/Ni/Au samples at anytemperature experienced short circuits.

Examining the effects of electrode fabrication methods, comparing r.f.plasma-assisted sputtering and DC sputtering, none of the samplesfabricated by r.f. plasma-assisted sputtering experienced shortcircuiting. By r.f. plasma-assisted sputtering, the probability of noshort circuits was improved.

The adhesion strength of AlN thin films was evaluated by a scratch testwith a scratch tester. A peel-off load of 169.2 mN was obtained. The AlNthin film was confirmed to have a relatively high adhesion strength.

To apply the present example to the development of mass productiontechnology for aluminum nitride thin films, we reviewed the fabricationof high-oriented aluminum nitride (AlN) thin films, measures whichprevent cracks and other factors from leading to short circuits, theimprovement of sensitivity of thin films, and the identification of massproduction deposition conditions. As a result, by optimizing the typeand stack structure of the bottom electrode, a superhigh orientation AlNthin film with a rocking curve full width half maximum of 0.4° wasfabricated on a glass substrate. In addition, by optimizing theelectrode fabrication temperature and fabricating the bottom electrodeby r.f. plasma-assisted sputtering to fabricate thin films with no shortcircuits, the probability of short circuits could be reduced.

As a result, the effects of the fabrication temperature differeddepending on the stack structure of the electrode thin film. None ofCr/Pt samples fabricated at 300° C., Ti/Pt/Au samples at roomtemperature, and Cr/Ni/Au samples at any temperature experienced shortcircuits. Fabrication by r.f. plasma-assisted sputtering could reducethe probability of short circuits. In addition, measurements of adhesionstrength between the AlN thin film and the glass substrate revealed arelatively high strength.

It is preferable in a piezoelectric element based on thesuperhigh-oriented aluminum nitride thin film if the bottom electrodecontains either two layers of Ti/Pt or Cr/Pt or three layers ofTi/Pt/Au, Ti/Ni/Au, or Cr/Ni/Au. The notation “A/B” indicates that themetal A sits on the substrate, that is, the first layer, and that themetal B sits on the metal A, that is, the second layer. The notation“A/B/C” indicates that the metal A sits on the substrate, that is, thefirst layer, that the metal B sits on the metal A, that is, the secondlayer, and that the metal C sits on the metal B, that is, the thirdlayer.

It is preferable in a piezoelectric element based on thesuperhigh-oriented aluminum nitride thin film if the substrate is aglass substrate.

It is preferable in a manufacturing method of a piezoelectric elementbased on the superhigh-oriented aluminum nitride thin film if a glasssubstrate is used as the substrate.

It is preferable in a manufacturing method of a piezoelectric elementbased on the superhigh-oriented aluminum nitride thin film if the bottomelectrode is deposited by r.f. plasma-assisted sputtering.

The embodiments and examples described in Best Mode for Carrying Out theInvention are for illustrative purposes only and by no means limit thescope of the present invention. Variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the claims.

INDUSTRIAL APPLICABILITY

Despite using glass or other cheap substrates, the present invention hassuccessfully provided a high performance piezoelectric element byforming a bottom electrode from a W layer on the substrate without anintervening adhesive layer. The invention further achieved both highperformance and high quality. “High quality” refers to the absence ofhillocks, cracks, and peeling. “High performance” means that thepiezoelectric element of the invention is substantially equivalent topiezoelectric elements on monocrystal substrates. The significance ofthe present invention is that it has made it possible to providepiezoelectric elements based on high performance, high quality aluminumnitride thin films formed on glass and other cheap substrates. Further,the present invention has successfully provided a high performance, highquality piezoelectric element on cheap substrates by selecting asuitable material for the surface layer of the bottom electrode informing not only a single W layer, but also a bottom electrode which isa stack body including an adhesive layer. Further, the present inventionhas successfully provided a manufacturing method for the highperformance, high quality piezoelectric elements which causes nohillocks, cracks, or peeling, by controlling particle shape and inaddition depositing by r.f. plasma-assisted sputtering.

The present invention can form aluminum nitride thin films withsuperhigh c-axis orientation not only on monocrystal substrates, butalso on quartz glass and other cheap and numerous kinds of substrates,by considering the matching of the crystal structures of the bottomelectrode and the aluminum nitride to form a suitable surface crystalstructure as an attempt to eliminate substrate factors disrupting theorientation of the aluminum nitride. Therefore, the invention givesfreedom in configuration and design of the piezoelectric element and ishighly applicable.

1. A piezoelectric element using a superhigh-oriented aluminum nitridethin film, the piezoelectric element being free from hillocks, cracks,and peeling and including a stack structure in which a bottom electrode,a piezoelectric body thin film, and a top electrode are sequentiallyformed on a substrate, the bottom electrode being made of an oriented Wlayer of which a (111) plane of W is parallel to a surface of thesubstrate, and the piezoelectric body thin film being formed of ac-axis-oriented aluminum nitride thin film having a rocking curve fullwidth half maximum (RCFWHM) not exceeding 2.5°.
 2. The piezoelectricelement using a superhigh-oriented aluminum nitride thin film as setforth in claim 1, wherein the substrate is a glass substrate.
 3. Apiezoelectric element using a superhigh-oriented aluminum nitride thinfilm, the piezoelectric element being free from hillocks, cracks, andpeeling and including a stack structure in which a bottom electrode, apiezoelectric body thin film, and a top electrode are sequentiallyformed on a substrate, the bottom electrode containing as a bottom layeran adhesive layer adhering to the substrate, the bottom electrode beingmade of a stack body, the stack body having a surface layer made of ametal layer having an electronegativity of around 1.4 and such anorientation that a crystal plane of a metal having an identical atomicconfiguration to an atomic configuration on a (001) plane of aluminumnitride and an almost equal atomic distance to an atomic distance on the(001) plane is parallel to a surface of the substrate, and thepiezoelectric body thin film being formed of a c-axis-oriented aluminumnitride thin film having a rocking curve full width half maximum(RCFWHM) not exceeding 2.5°.
 4. The piezoelectric element using asuperhigh-oriented aluminum nitride thin film as set forth in claim 3,wherein the substrate is a glass substrate.
 5. A piezoelectric elementusing a superhigh-oriented aluminum nitride thin film, the piezoelectricelement being free from hillocks, cracks, and peeling and including astack structure in which a bottom electrode, a piezoelectric body thinfilm, and a top electrode are sequentially formed on a substrate, thebottom electrode containing as a bottom layer an adhesive layer adheringto the substrate, the bottom electrode being made a stack bodycontaining as a surface layer such an oriented W, Pt, Au, or Ag layerthat a (111) plane of W, Pt, Au, or Ag is parallel to a surface of thesubstrate, and the piezoelectric body thin film being formed of ac-axis-oriented aluminum nitride thin film having a rocking curve fullwidth half maximum (RCFWHM) not exceeding 2.5°.
 6. The piezoelectricelement using a superhigh-oriented aluminum nitride thin film as setforth in claim 5, wherein the bottom electrode is made up of either twolayers of Ti/Pt or Cr/Pt in accordance with a notation, “the first layerformed on the substrate/the second layer formed on the first layer” orthree layers of Ti/Pt/Au, Ti/Ni/Au, or Cr/Ni/Au in accordance with anotation, “the first layer formed on the substrate/the second layerformed on the first layer/the third layer formed on the second layer.”7. The piezoelectric element using a superhigh-oriented aluminum nitridethin film as set forth in claim 5, wherein the substrate is a glasssubstrate.
 8. A method of manufacturing a piezoelectric element using asuperhigh-oriented aluminum nitride thin film, the method comprising thesequential steps of: forming a bottom electrode on a substrate from suchan oriented W layer that a (111) plane of W is parallel to a surface ofthe substrate by sputtering at a temperature from room temperature to alow temperature at which no spaces develop between W particles; andforming a piezoelectric body thin film of a c-axis-oriented aluminumnitride thin film having a rocking curve full width half maximum(RCFWHM) not exceeding 2.5° on the bottom electrode; and forming a topelectrode on the piezoelectric body thin film.
 9. The method ofmanufacturing a piezoelectric element using a superhigh-orientedaluminum nitride thin film as set forth in claim 8, wherein thesubstrate is a glass substrate.
 10. The method of manufacturing apiezoelectric element using a superhigh-oriented aluminum nitride thinfilm as set forth in claim 8, wherein the bottom electrode is depositedby r.f. plasma-assisted sputtering.
 11. A method of manufacturing apiezoelectric element using a superhigh-oriented aluminum nitride thinfilm, the method comprising the sequential steps of: in forming, on asubstrate, a bottom electrode of a two- or more-layered stack structureincluding an adhesive layer adhering to the substrate, firstlydepositing the adhesive layer by sputtering at a temperature from roomtemperature to a low temperature at which no spaces develop betweenparticles and then depositing as a surface layer of the bottom electrodea metal layer by sputtering at a temperature from room temperature to alow temperature at which no spaces develop between particles so that themetal layer exhibits such orientation that a crystal plane of a metal isparallel to a surface of the substrate, by using such a metal having anelectronegativity of around 1.4 that a crystal plane of the metal has anidentical atomic configuration to an atomic configuration on a (001)plane of aluminum nitride and an almost equal atomic distance to anatomic distance on the (001) plane; forming a piezoelectric body thinfilm of a c-axis-oriented aluminum nitride thin film having a rockingcurve full width half maximum (RCFWHM) not exceeding 2.5° on the bottomelectrode; and forming a top electrode on the piezoelectric body thinfilm.
 12. The method of manufacturing a piezoelectric element using asuperhigh-oriented aluminum nitride thin film as set forth in claim 11,wherein the substrate is a glass substrate.
 13. The method ofmanufacturing a piezoelectric element using a superhigh-orientedaluminum nitride thin film as set forth in claim 11, wherein the bottomelectrode is deposited by r.f. plasma-assisted sputtering.
 14. A methodof manufacturing a piezoelectric element using a superhigh-orientedaluminum nitride thin film, the method comprising the sequential stepsof: in forming, on a substrate, a bottom electrode of a two- ormore-layered stack structure including an adhesive layer adhering to thesubstrate, firstly depositing the adhesive layer by sputtering at atemperature from room temperature to a low temperature at which nospaces develop between particles and then depositing as a surface layeran oriented W, Pt, Au, or Ag layer that a (111) plane of W, Pt, Au, orAg is parallel to a surface of the substrate by sputtering at atemperature from room temperature to a low temperature at which nospaces develop between particles; forming a piezoelectric body thin filmof a c-axis-oriented aluminum nitride thin film having a rocking curvefull width half maximum (RCFWHM) not exceeding 2.5° on the bottomelectrode; and forming a top electrode on the piezoelectric body thinfilm.
 15. The method of manufacturing a piezoelectric element using asuperhigh-oriented aluminum nitride thin film as set forth in claim 14,wherein the substrate is a glass substrate.
 16. The method ofmanufacturing a piezoelectric element using a superhigh-orientedaluminum nitride thin film as set forth in claim 14, wherein the bottomelectrode is deposited by r.f. plasma-assisted sputtering.
 17. Apiezoelectric element using a superhigh-oriented aluminum nitride thinfilm, the piezoelectric element including a bottom electrode, apiezoelectric body thin film of aluminum nitride, and a top electrodestacked in this order on a substrate; and the aluminum nitride having arocking curve (RCFWHM) not exceeding 2.5°.
 18. The piezoelectric elementusing a superhigh-oriented aluminum nitride thin film as set forth inclaim 17, wherein the bottom electrode is either a single, metal layeror a stack body including an adhesive layer adhering to the substrateand one or more metal layers on the adhesive layer.
 19. Thepiezoelectric element using a superhigh-oriented aluminum nitride thinfilm as set forth in claim 18, wherein the stack body has a surfacelayer made of a metal having an electronegativity between 1.3 and 1.5inclusive.
 20. The piezoelectric element using a superhigh-orientedaluminum nitride thin film as set forth in claim 18, wherein the stackbody has a surface layer made of a metal having a crystal plane havingan identical atomic configuration to an atomic configuration on a (001)plane of aluminum nitride and an almost equal atomic distance to anatomic distance on the (001) plane.
 21. The piezoelectric element usinga superhigh-oriented aluminum nitride thin film as set forth in claim18, wherein the stack body has as a surface layer an oriented W, Pt, Au,or Ag layer that a (111) plane of W, Pt, Au, or Ag is parallel to asurface of the substrate.
 22. The piezoelectric element using asuperhigh-oriented aluminum nitride thin film as set forth in claim 18,wherein the stack body is made up of either two layers of Ti/Pt or Cr/Ptin accordance with a notation, “the first layer formed on thesubstrate/the second layer formed on the first layer” or three layers ofTi/Pt/Au, Ti/Ni/Au, or Cr/Ni/Au in accordance with a notation, “thefirst layer formed on the substrate/the second layer formed on the firstlayer/the third layer formed on the second layer.”
 23. The piezoelectricelement using a superhigh-oriented aluminum nitride thin film as setforth in claim 17, wherein the substrate is a glass substrate.